Finally, the halo concentration can be estimated using the velocity of member galaxies as tracers of the underlying mass distribution. This was done in [1]}, were the authors analysed a sample of \(\sim 100\)  clusters and groups, dividing them in different magnitude-gap bins. For each bin, they stacked all the available galaxies to increase the statistic. They found \(c_{200} = 2.5 \pm 0.4\)  for the bin with the largest magnitude gap (defined as \(\Delta m_{12} > 1.5\) ). This values are in agreement, within the uncertainties, with their results in the other three magnitude-gap bins, as well as with other similar work in the literature of non-fossil clusters [2]}, [3]}. However, the large uncertainties typical of this observational technique prevented the authors from reaching a strong conclusion.
